This invention relates to a system for generating a local oscillator signal in a digital demodulator of the type comprising means for receiving an input signal modulated with digital data, means for multiplying the input signal with a local oscillator signal to generate a product signal, and means for periodically integrating the product signal to generate a sequence of integrated signals, each having an amplitude indicative of a respective portion of the digital data.
Digital demodulators of the general type described above are well known to those skilled in the art, as for example in quadrature amplitude modulation (QAM) systems. Typically, a plurality of separate component signals are combined to form a composite signal which is transmitted on a single signal channel, and each signal is orthogonal to the others and is amplitude modulated. When the received composite signal is multiplied by an appropriate local oscillator signal and the resulting product is integrated over an integral number of symbol periods, the resulting integration is indicative of the amplitude of a respective one of the component signals of the composite signal.
Preferably, each of the component signals is amplitude modulated with a sine wave of half the data symbol rate, phased such that the amplitude is zero at the beginning and end of each symbol. This reduces the spectrum created by the data transitions on the symbol boundaries. For this reason, the local oscillator signals are also often sinusoidally varying.
In the past, such sinusoidally varying local oscillator signals have been generated by integrating square wave signals. This approach yields an adequate approximation of a sine wave signal; however, it suffers from the disadvantage that the phase of the local oscillator signal cannot be reset at high speed, because the local oscillator signal is generated in an integration operation. In many applications it is important that a digital demodulator of the type described above be able to shift the phase of the local oscillator signal rapidly, as for example when an input signal of previously unknown phase is acquired. In these situations, the integration approach to generating a local oscillator signal may unacceptably reduce the speed with which the input signal can be acquired.
Another approach of the prior art is to store a desired local oscillator signal in read only memory and then to apply the output of the read only memory to a digital to analog converter for conversion to an analog signal that is used as the local oscillator. The use of such a read only memory allows the phase of the local oscillator signal to be changed rapidly. However, this approach requires hardware which is relatively high-speed and complex.
A need presently exists for an improved apparatus for generating a local oscillator signal in a digital demodulator of the type described above, which allows the phase angle of the local oscillator signal to be adjusted rapidly, and which can be implemented simply and economically. In many applications it is important that such a local oscillator signal be shaped to ensure that third and fourth harmonic components are substantially reduced in amplitude as compared with the fundamental component of the local oscillator signal.